Plasticity of vertical distribution of crustacean zooplankton in lakes with varying levels of water colour

Zooplankton that are vulnerable to predation by planktivorousfish typically remain in deeper, darker water during the day.Vertical position may be affected by profiles of temperatureand oxygen concentration, but light levels are thought to becritically important. Water transparency in temperate lakesis strongly dependent on the concentration of dissolved organiccarbon (DOC). Therefore, different patterns of zooplankton verticalposition might be expected in lakes that vary in their levelof DOC. The importance of three factors (light, temperatureand oxygen) that might affect vertical position of major zooplanktongroups was evaluated in 10 small lakes located in central Ontario.The lakes, encompassing a wide range of water colour, were sampledin May, June and August of 2000. Small cladocerans were foundto be always higher in the water column than large cladocerans,while copepods were deeper. Small cladocerans had a weak responseto the predictive factors, but copepods and large cladoceranswere strongly affected. The vertical position of copepods wasconsistently dependent solely on Secchi depth. Surprisingly,for large cladocerans, temperature gradient was decisive inMay and June, and only in August was water transparency themost important variable. Overall, water transparency, as determinedby level of water colour, was the most important variable but,depending on taxonomic group and time of year, temperature andoxygen were also major determinants of vertical position.